Circular dichroism (CD) finds widespread application as an optical probe for the structure of molecules and supramolecular assemblies. Its underlying chiral light−matter interactions effectively couple between photonic spin states and select quantum-mechanical degrees of freedom in a sample, implying an intricate connection with photon-to-matter quantum transduction. However, effective transduction implementations likely require interactions that are antisymmetric with respect to the direction of light propagation through the sample, yielding an inversion of the chiroptical response upon sample flipping, which is uncommon for CD. Recent experiments on organic thin films have demonstrated such chiroptical behavior, which was attributed to "apparent CD" resulting from an interference between the sample's linear birefringence and linear dichroism. However, a theory connecting the underlying optical selection rules to the microscopic electronic structure of the constituent molecules remains to be formulated. Here, we present such a theory based on a combination of Mueller calculus and a Lorentz oscillator model. The theory reaches good agreement with experimental CD spectra and allows for establishing the (supra)molecular design rules for maximizing or minimizing this chiroptical effect. It furthermore highlights that, in addition to antisymmetrically, it can manifest symmetrically such that no chiroptical response inversion occurs, which is a consequence of a helical stacking of molecules in the light propagation direction.

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Breaking the symmetry between left-handed and right-handed chiral optical modes in planar Fabry–Pérot (FP) microcavities would enable a variety of chiral light-matter phenomena, with applications in spintronics, polaritonics, and chiral lasing. Such symmetry breaking, however, has remained underexplored and has been purported to require Faraday mirrors. We present a simple solution to chiral symmetry breaking in FP microcavities, preserving low mode volumes by embedding organic thin films exhibiting "apparent circular dichroism" (ACD); an optical phenomenon based on interfering linear birefringence and linear dichroism with offset optical axes. ACD interactions are opposite for counter-propagating light and increase with path length. Consequently, we demonstrated chiral asymmetry of the cavity modes over an order of magnitude larger than that of the isolated thin film. Through both circular dichroism spectroscopy and simulation using theoretical scattering matrix methods, we characterize the spatial, spectral, and angular chiroptical responses of this new type of chiral microcavity.